///////////////////////////////////////////////////////////////////////////
//                                                                       //
// Program file name: Dart.java                                          //
//                                                                       //
// © Tao Pang 2006                                                       //
//                                                                       //
// Last modified: January 18, 2006                                       //
//                                                                       //
// (1) This Java program is part of the book, "An Introduction to        //
//     Computational Physics, 2nd Edition," written by Tao Pang and      //
//     published by Cambridge University Press on January 19, 2006.      //
//                                                                       //
// (2) No warranties, express or implied, are made for this program.     //
//                                                                       //
///////////////////////////////////////////////////////////////////////////

// An example of evaluating pi by throwing a dart to a
// unit square with 0<x<1 and 0<y<1.

import java.lang.*;
import java.util.Calendar;
import java.util.GregorianCalendar;
public class Dart {
  final static int n = 1000000;
  static int seed;
  public static void main(String argv[]) {

 // Initiate the seed from the current time
    GregorianCalendar t = new GregorianCalendar();
    int t1 = t.get(Calendar.SECOND);
    int t2 = t.get(Calendar.MINUTE);
    int t3 = t.get(Calendar.HOUR_OF_DAY);
    int t4 = t.get(Calendar.DAY_OF_MONTH);
    int t5 = t.get(Calendar.MONTH) + 1;
    int t6 = t.get(Calendar.YEAR);
    seed = t6+70*(t5+12*(t4+31*(t3+23*(t2+59*t1))));
    if ((seed%2) == 0) seed = seed-1;

 // Throw the dart to the unit square
    int ic = 0;
    for (int i=0; i<n; ++i) {
      double x = ranf();
      double y = ranf();
      if ((x*x+y*y) < 1) ic++;
    }
    System.out.println("Estimated pi: " + (4.0*ic/n));
  }

// Method to generate a uniform random number in [0,1]
// following x(i+1)=a*x(i) mod c with a=pow(7,5) and
// c=pow(2,31)-1.  Here the seed is a global variable.

  public static double ranf() {
    final int a = 16807, c = 2147483647, q = 127773,
      r = 2836;
    final double cd = c;
    int h = seed/q;
    int l = seed%q;
    int t = a*l-r*h;
    if (t > 0) seed = t;
    else seed = c+t;
    return seed/cd;
  }
}